1. Field of the Invention
The invention relates to an image reading apparatus such as scanner, facsimile, or the like.
2. Related Background Art
As an illuminating apparatus which is used in an image reading apparatus such as scanner, facsimile, or the like, an apparatus with a structure as shown in FIGS. 1A and 1B has been proposed. FIG. 1A shows a cross sectional view of the illuminating apparatus and FIG. 1B is a diagram when it is seen from the longitudinal direction. As shown in the diagram, in the illuminating apparatus, an LED 7 of a lead frame type is used as a light source. The LEDs 7 are arranged at both edges of an optical guide 8 for guiding an irradiation light. FIG. 2 is an internal constructional diagram for explaining an internal structure of the illuminating apparatus in detail. In FIG. 2, the optical guide 8 guides, in the longitudinal direction. the irradiation light of the LED 7 which is lit on by being supplied with an electric power by lead wires 11. The guided light repeats the internal reflection in the optical guide 8. The light is reflected by a diffusing surface 10 provided in the longitudinal direction of the optical guide 8 and irradiates an original to be read in a line shape.
The LEDs 7 and optical guide 8 are positioned and held by a housing 9. An opening portion (slit) 12 corresponding to a length of original is formed in the housing 9 so that the light reflected by the diffusing surface 10 irradiates the original to be read. In order to reduce the whole length of illuminating apparatus as short as possible and to assure an effective length adapted to read the original as shown in FIG. 1B, the length of opening portion 12 is not limited to the length of optical guide 8 but the opening portion 12 is extended to a position where the LEDs 7 as light sources can be directly seen, thereby allowing the direct lights from the light sources to be contributed to the illumination of the original.
However, when it is tried to irradiate the original by using the direct light of the LED light source, there are the following problems.
(1) As shown in FIG. 3, since an illuminance of a portion where the direct light of the light source is irradiated is remarkably higher than that of a portion where the indirect light from the optical guide is irradiated, a variation in illuminance occurs on a reading line of the original. When an illuminance deviation increases as mentioned above, when the read image is outputted by an image output apparatus, a problem on a picture quality occurs.
(2) To eliminate such an illuminance deviation, if the light source is shut off so that the direct light of the light source does not irradiate the original, the light source portion becomes a dead space on the illuminating apparatus, so that an illumination width enough to irradiate the original cannot be assured.
(3) When it is intended to mechanically assure the original illumination width, a whole length of illuminating apparatus becomes remarkably long, so that a size of image reading apparatus is enlarged.